In recent years, an image reading system such as a fax machine, a copier or a scanner is equipped with a duplex document conveying apparatus which reads image information printed or drawn on both surfaces of a document. The duplex document conveying apparatus conveys the document automatically without turning front to back surface. The duplex document conveying apparatus comprises a conveying path for reading a front surface (a first surface) and that for reading a back surface (a second surface). Because of a demand for downsizing, in many of the duplex document conveying apparatuses, a part of the conveying path for reading a first surface is shared for reading a second surface.
In such a duplex document conveying apparatus, when a document elongated in a conveying direction is conveyed, there sometimes occurs overlapping of the document conveyances at the shared part of the conveying path. That is, two parts of the document, which are conveyed in directions different from each other, pass the same conveying path, simultaneously. In a related art, in order to avoid a contact between the two parts of the document, conveying rollers at the shared part are positioned separately from each other, or a gap is provided between the conveying rollers.
FIG. 13 shows an essential portion of a duplex document conveying apparatus disclosed in Japanese Patent Application Laid-Open No. 1997-086807. In the duplex document conveying apparatus 100, documents (not illustrated), which are set in a document feeding tray 101 are picked one sheet by one sheet by a picking roller 102, and are sent out to a nip area of a pair of sheet feeding rollers 103 and 104. The pair of sheet feeding rollers 103 and 104 sends out the conveyed document in a direction of an arrow 106 along a guide member 105. In the vicinity of an end of the guide member 105, arranged is a pair of first conveying rollers 107 and 108. By rotation of the pair of first conveying rollers 107 and 108, the document moves along a curved guide member 109. A conveying direction of the document changes from the horizontal direction to a downward direction indicated by an arrow 111. The document is further guided to a nip area of a pair of second conveying rollers 112 and 113.
On the document discharging side of the pair of second conveying rollers 112 and 113, a transparent plate 114 such as a platen glass is arranged horizontally. Underneath the transparent plate 114, arranged is a reading mechanism (not illustrated), which comprises a linear image sensor or an optical system along a primary scanning direction of the document. The document is conveyed by rotation of the pair of second conveying rollers 112 and 113 and passes between the transparent plate 114 and a guide member 115 arranged above the transparent plate. When the document passes the transparent plate, a first surface of the document is sequentially read at a document reading position 116 of the reading mechanism.
The document, for which the reading at the document reading position 116 has been completed, passes between a pair of third conveying rollers 117 and 118, and then, being guided by a guide member 119, is conveyed in a direction toward a pair of discharge/reverse rollers 121 and 122. When only one surface (the first surface) of the document is to be read, the document passes between the pair of discharge/reverse rollers 121 and 122, and then, is conveyed in a direction of an arrow 123 and is discharged into a document discharge tray (not illustrated), which is arranged underneath the document feeding tray 101.
In contrast, in the case of a duplex reading mode where both the first and second surfaces of the document are to be read, the trailing edge of the document conveyed in the direction of the arrow 123 after passing between the pair of discharge/reverse rollers 121 and 122 is detected by a document detection sensor 124 arranged before the pair of discharge/reverse rollers 121 and 123. On the basis of the time at which the trailing edge is detected, a rotation direction of the pair of discharge/reverse rollers 121 and 122 is reversed.
By the reversal of the rotation direction of the discharge/reverse rollers 121 and 122, the part having been the trailing edge until this time turns to a leading edge, and conveyance of the document in a direction of an arrow 125 along the guide member 119 starts. Guided by a guide member 126 located above the third conveying roller 117, the document is conveyed toward a nip area of the pair of first conveying rollers 107 and 108. Then, after passing between the pair of first conveying rollers 107 and 108, the document passes above the transparent plate 114 with the second surface facing downward. At this time, the second surface of the document is read sequentially.
The document, both surfaces of which have been read, described as above, passes between the pair of discharge/reverse rollers 121 and 122, and then is discharged into the document discharge tray, in the same way as the case where only the first surface was read.
In the duplex document conveying apparatus 100, described as above, when the second surface of the document is read, the document is conveyed on a loop-like route which passes from the pair of discharge/reverse rollers 121 and 122, through a series of the guide members 126, 109, 115 and 119 sequentially, and returns again to the pair of discharge/reverse rollers 121 and 122. If the length of the document in the conveying direction is longer than the length of the loop-like route, there occurs a state where two parts of the same document simultaneously pass between the pair of discharge/reverse rollers 121 and 122 in directions opposite to each other. Accordingly, a material with low friction coefficient is required to be employed as roller surfaces of the pair of discharge/reverse rollers 121 and 122, or the rollers needed to be separated from each other at a predetermined timing to create a gap between the rollers so that the two parts of the document pass each other there.
In the duplex document conveying apparatus 100 of the related art, a linkage mechanism having first to third link members, 133 to 135, which move a rotary shaft of the discharge/reverse roller 121 upward and downward, is fixed to a plunger 132 of a solenoid 131. In FIG. 13, the third link member 135 supports a rotary shaft 136 of the upper discharge/reverse roller 121. The first and second link members 133 and 134 move the discharge/reverse roller 121 upward and downward via the link member 135 according to moving of the plunger 132 into and out from the solenoid.
FIG. 14 specifically shows a mechanism for switching a pressing force exerted on the document by means of the pair of discharge/reverse rollers. One end of the first link member 133 for extending a length of the plunger is fixed to the plunger 132. To the other end of the first link member 133, one end of the second link member 134 is rotatably fixed. The second link member 134 has an obtusely bent shape. At the position of the bending, a pin 141 is fixed onto an immovable member (not illustrated) in the direction perpendicular to the drawing.
To the other end of the second link member 134, one end part of the third link member 135 having an elongated hole 142 at the other end part is fixed. In the elongated hole 142 of the third link member 135, a rotary shaft 136 of the discharge/reverse roller 121 is slidably arranged.
At the time of reading the second surface, the document 144 passes between the pair of discharge/reverse rollers 121 and 122 and is conveyed in the direction of the arrow 125. A leading edge of the document 144 passes the pair of first conveying rollers 107 and 108, the pair of second conveying rollers 112 and 113, and the pair of third conveying rollers 117 and 118, which are shown in FIG. 13, and returns again to the nip area of the pair of discharge/reverse rollers 121 and 122.
At the moment just before the leading edge 144A of the document 144 passes the pair of third conveying rollers 117 and 118, the document detection sensor 124 shown in FIG. 13 detects an arrival of the leading edge of the document. Then, an excitation circuit (not illustrated) excites the solenoid 131, and the plunger 132 moves in a direction of an arrow 145, i.e. the direction of discharging the document 144. Then, the second link member 134 rotates by a predetermined angle counterclockwise with the pin 141 as a supporting point. As a result, the discharge/reverse roller 121 on the driven side moves diagonally upward as shown by an arrow 146 in FIG. 14, thereby separates from the discharge/reverse roller 122 on the driving side.
As described above, while the document 144 passes between the pair of discharge/reverse rollers 121 and 122 in a “twofold” state, a pressing force exerted on the document 144 by these rollers is weakened. If a trailing edge of the document 144 has passed between the pair of discharge/reverse rollers 121 and 122, the excitation circuit (not illustrated) ends exciting the solenoid 131. Then, the plunger 132 returns to the original position by means of a spring (not illustrated). The pair of discharge/reverse rollers 121 and 122 pressingly contact the document 144 in a “onefold” state conveyed in the discharging direction from both sides of the document. As a result, thereafter, the pair of discharge/reverse rollers 121 and 122 conveys the document 144 in the discharging direction.
However, in the duplex document conveying apparatus 100 according to the related art shown in FIG. 13, the driving component such as the solenoid 131 is mounted, and the electric circuit forcibly separates the pair of discharge/reverse rollers 121 and 122 from each other during the specific period of time. Accordingly, the duplex document conveying apparatus 100 needs a region for mounting the above-described driving component and an electric circuit to control the driving component. Such a region is an obstacle to downsizing of the apparatus. Additionally, replacement of these components on their trouble increases the cost, then the operation rate of an image reading apparatus decreases.
In another related art, mutual pressing forces between the pair of discharge/reverse rollers 121 and 122 are weakened. At the time when the leading edge part of and the trailing edge part of the document pass each other, if the conveyed document has high rigidity, a gap between the rollers is forcibly widened by the rigidity of the document. As a result, the trouble due to the mutual contact between the two parts of the document in the “twofold” state, when the leading edge part and the trailing edge part pass each other, is reduced.
However, in this another related art, there arises a problem that the conveying force exerted on the document by the pair of discharge/reverse rollers 121 and 122 is insufficient when the document is in the “onefold” state. The pair of discharge/reverse rollers 121 and 122 has a first function to forcibly discharge the document into the discharge tray and a second function, at the time of reading the second surface in the duplex reading mode, to draw the document having moved to the discharge tray side back to the inside of the main body of the apparatus.
If these first and second functions become insufficient, during the period when an operation of the pair of discharge/reverse rollers 121 and 122 contributes to the conveying of the document, a conveying speed of the document at the document reading position 116 becomes unstable. At that time, there occurs a trouble in that an image reading operation in a secondary scanning direction becomes unstable and the image quality is accordingly degraded.
Further, when downsizing the duplex document conveying apparatus 100, the curved guide member 109 needs to have a bus shape with a large curvature. A conveying load from the document conveyed on such a guide member 109 is so large that the conveying force by the pair of discharge/reverse rollers 121 and 122 is lowered. Therefore, the duplex document conveying apparatus 100 according to the another related art is not suitable for downsizing.